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Levels of Muscle Structure

• Muscle• Muscle Bundle• Muscle Fiber

(myofiber)• Myofibrils• Myofilaments

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Muscle

Muscle Bundle• Contains 10-20

myofibers• Encased by

perimysium• Can see with the

naked eye

Myofiber (Muscle fiber)• Individual muscle

cell• Multinucleated• Encased by

endomysium• Cell wall:

sarcolemma

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Myofibrils• Embedded in

sarcoplasm• Mitochondria

located between myofibrils

Myofibrils

• Comprised of repeating units: sarcomeres– A band– I band– Z disk– H zone– Pseudo-H zone

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Sarcomere

Sarcomere

Myofilaments

• Contractile Proteins– Myosin– Actin

• Regulatory Proteins– Tropomyosin– Troponin

• Structural Proteins– Z – Line Proteins

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Contractile Proteins• Myosin

– 70 – 80% of the total protein

– Thick filament– Burns the ATP for

muscle contraction– Myosin head moves

back and forth to perform a muscle contraction

My osin & Its Fragments

COO-

COO-L MM

RO D

S1

H MM

Reg ula tory light chainAlkal il ight chain

Act in bindingand A TPa se

activity

Ch ymotrypsin

Trypsin

Contractile Proteins• Actin

– 20% of the myofibrillar protein

– Thin Filament – Globular protein (G-

protein)– Arranged like a

twisted pearl necklace (F-protein)

– Myosin head attaches to the Actin

Regulatory Proteins

• Regulate contraction and the speed of contraction

• Tropomyosin• Troponin

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Regulatory Proteins

• Tropomyosin– Thin protein that lays around the Actin

proteins

A A

End ViewTop View

Regulatory Proteins• Troponin • 3 Subunits

– TnT• Binds tropomyosin

– TnI• Inhibitory subunit

– TnC• Ca2+ binding subunit

3 Dimensional

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How does all this fit together?

More Structural Stuff• Sarcolemma – membrane around the

myofibril; sits just under the endomysium

• Sarcoplasmic Reticulum• T – tubules or Transverse Tubules• Terminal Cisternae

More Structural Stuff

• Sarcoplasmic Reticulum – Surrounds each myofibril – Stores Calcium, needed for contraction

• T – tubules and Terminal Cisternaetransport Ca to cytosol & transmit nerve impulse

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We’ve laid the ground work, let’s talk about muscle contraction

“Sliding Rod Theory” Hanson and Huxley 1955; Huxley 1965, 1972; Huxley and Hanson, 1960

Muscle Contraction

• A signal travels down a nerve

• Attached to individual muscle cells

• The signal is passed on to the Sarcolemma

• The Sarcolemmadepolarizes

Muscle Contraction

• The depolarization causes the SR to release Ca into the cytosol

• The Ca will bind with troponin (Tn)• This causes a shift in the troponin

tropomyosin (Tm) complex

Actin Actin Actin Actin

Ca

Ca Ca

CaTn

Tn Tn

TnTm

TmTm

Tm

Myosin binding site

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The ATPase activity of Myosin

The ATPase activity of Myosin• ATP (Adenosine Triphosphate) bind to the

Myosin head• ATP hydrolysis to ADP + Pi “cocks” the

Myosin head

The ATPase activity of Myosin

• The Myosin head attaches to the exposed binding site on Actin

• Weak bond• Pi leaves the Myosin

head causing the “Power Stroke”

• ADP is released causing a strong Myosin – Actin bond

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The ATPase activity of Myosin

• ATP re-attaches to the Myosin head causing the head to release from the Myosin – Actinbinding site

Let’s put it all together

• 1.) An impulse travels down a nerve to a muscle cell

• 2.) The nerve impulse is transferred to the Sarcolemma of a muscle cell

• 3.) The Sarcolemma depolarized causing the Sarcoplasmic Reticulum to release Ca into the cytosol of the cell

Let’s put it all together

• 4.) The Ca binds to Troponin on the Troponin – Tropomyosin complex

• 5.) The Tn – Tm complex shifts to the grove of the Actin exposing the Actin –Myosin binding site

• 6.) ATP has bound with the Myosin head releasing it from the previous contraction

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Let’s put it all together

• 7.) Myosin hydrolyzes the ATP to ADP + Pi, “cocking” the Myosin head

• 8.) Myosin attaches to Actin forming a weak bond

• 9.) Pi is released causing the “power stroke”

• 10.) ADP is released forming a tight rigor bond of Actin and Myosin

http://www.tvermilye.com/pmwiki/pmwiki.php?n=Animation.Video12

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What makes these guys so strong?

What makes those guys strong is the same that makes him strong!

Or, Who’s Stronger??

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See ya this afternoon